1. Field of Invention
This invention relates to an electrode roll for a welding machine in which a side seam of overlapped metal sheet of a can body is formed by electric resistance seam welding. More specifically, the invention concerns an electrode roll in which a fusible and conductive liquid alloy composed mainly of gallium is sealed in a gap formed between a stator and a rotor.
2. Description of the Prior Art
Up to date, a side seam of overlapped portions of a can body, i.e., a blank of such metal sheet as tin- or nickel-plated steel sheet set in a cylindrical form, is formed by an electric resistance seam welding process. In this process, the overlapped portions of the can body are squeezed via a copper wire electrode between electrode rolls on their inner and outer sides while current is passing through them.
The electrode roll used in an electric resistance seam welding process as shown in FIG. 1, has a rotor 102 rotatably mounted on the periphery of stator 100 via an insulated bearing 101.
A disk-like portion 100a is formed in axial center position of the stator 100. There is a small gap between the outer periphery of the disk-like portion 100a and inner surface of rotor 102.
The gap formed between outer periphery of the disk-like portion 100a and inner surface of rotor 102 is filled with conductive liquid metal 103.
In order to let welding current flow between stator 100 and rotor 102 via the conductive liquid metal 103, the conductive metal stator and rotor have copper or copper alloy parts which face the gap.
The conductive liquid metal 103 filling the gap of the electrode roll is described here.
Usually, mercury is widely used as the conductive liquid metal of the electrode roll. The reason is that the mercury has a very low melting point of -39.degree. C., and it maintains the liquid phase not only at normal temperature but also at a considerably lower temperature.
However, mercury has considerably strong toxicity, and therefore, a substituted material is desired.
As a result of extensive studies by the inventors, it has been found that a four-element alloy composed of 67 wt % of gallium, 20 wt % of indium, 10 wt % of tin and 3 wt % of zinc has a solidifying point of 6.5.degree. C. and a melting point of 8.5.degree. C. as well as having a resistivity of 22 .mu..OMEGA..cm, i.e., more than four times the conductivity of mercury whose resistivity is 95.8 .mu..OMEGA..cm.
Other fusible alloys composed mainly of gallium and used as conductive liquid metal for electrode roll are, for example, the six-element fusible alloys (Ga, In, Sn, Zn, Ag, Al) and the binary alloy (Ga, In) proposed (Japanese Patent Publication No. 40355/1980, No. 40359/1980 and Japanese Patent laid-open Publication No. 62680/1981, No. 77076/1981), are partly used.
However, in case the fusible liquid alloy composed mainly of gallium is used as the conductive liquid metal in the electrode roll, its melting point tends to rise and to make itself become easier to solidify because gallium strongly attacks copper and copper alloy and corrodes copper and copper alloy forming a rotor and a stator so that such copper and copper alloy are dissolved into the liquid metal when the electrode is in use. The resultant problem is that life of the electrode roll will he shortened very badly.
In order to solve the problem the applicants have proposed before (Patent Application No. 278822/1984) to plate the rotor and stator surfaces facing the gap with a tungsten-cobalt alloy which is harder to be corroded by the fusible alloy.
A filling hole is provided in the rotor for filling conductive liquid metal into the electrode roll. The inner surface of the filling hole is harder to be covered with a complete protective film of metal or metal alloy provided by means of electroplating etc., as compared with the rotor inner face other than this portion. Therefore, if there is any portion in the inner surface of the filling hole, which is not adequately covered with such protective film, corrosion attacks will concentrate in such portion.
When concentrated corrosion happens, copper in the rotor melts and is introduced into the fusible liquid alloy, causing the melting temperature of the fusible alloy to rise and the fusible alloy to solidify easier. As a result, the life of the electrode roll is shortened.
The rate of such concentrated corrosion relates to the location of the filling hole and a tendency is such that the corrosion develops more badly when the filling hole is provided at such a location as shown in FIG. 1, where welding current is larger than the hole is provided in a portion where welding current is small.